Emitting display devices such as liquid crystal displays (LCDs), cathode ray tubes (CRTs), organic light emitting diodes (OLEDs) and plasma display panels (PDPs) are present in a number of electronic devices among which are watches, mobile phones, displays in cars, monitors or TVs for example.
Those display devices are not energy efficient. The emission of light is the most important factor of power consumption in a display. Most of this light, emitted in all directions, is wasted because a very small amount reaches the eyes of a user.
Efforts have been made to control the direction of light on axis with the aim of saving power. The user is generally in front of the display, especially with handheld devices. It is sometimes not necessary to have sideways emission of light. Optical components have been designed to concentrate the light emitted by the light sources in a direction close to the axis of the display. Examples of these power saving components are prismatic structures in backlights for LCDs to collimate the light on axis (U.S. Pat. No. 6,967,698) or prismatic films known as brightness enhancement films (U.S. Pat. No. 4,542,449, U.S. Pat. No. 5,175,030) or arrays of lenses (U.S. Pat. No. 6,570,324 array of lenses on OLED). Those methods provide an increased efficiency on axis at the expense of a low visibility off axis. In those systems, the illumination profile does not change when the user is off axis. The user must be in front of the display if he wants to keep watching the display in optimal conditions. When he is off axis, he sees a low brightness image and the light on axis is wasted.
Directional displays such as 3D displays or privacy displays (US20050243265, U.S. Pat. No. 5,132,839) provide control of the light direction. These displays control this light direction by either redirecting the light beams or by absorbing the light emitted toward the unwanted direction.
3D displays send two different images into two adjacent viewing zones, one for each eye of the user. Some of these displays provide an illumination which is dependent on the user position. They use a tracking system that detects the position of the user. The light transmitted is directed to the two viewing windows providing a different image to each of these windows. The selection is realised by different optical means such as parallax barriers or arrays of microlenses. Some examples are U.S. Pat. No. 5,132,839 (Travis), US application 20050117016 (Surman). The refraction method, such as the lenticular system, provides an efficient way to control the light direction, not wasting light by absorption. However, in those display systems, crosstalk is an important parameter to avoid. Hence, when designing 3D or multi-view displays, the priority is to reduce the crosstalk but the power saving is not an important parameter.
3D displays must show different images to different locations in order to achieve the desired 3D effect. Privacy displays are “single view” displays in that they do not do this. However, privacy displays (US20050243265, U.S. Pat. No. 5,831,698, U.S. Pat. No. 6,211,930 U.S. Pat. No. 5,825,436 and U.S. Pat. No. 5,877,829) provide an image within a narrow viewing angle in order to prevent snooping from anybody out of this viewing zone. The viewing zone can be switched to a wider viewing zone in public mode, allowing for multiple users to watch the screen. The switching is controlled by the user. The downside of this system is that the user has to be in front of the screen. The narrow viewing zone does not follow the main user position.
The downside of these directional displays comes from the requirement of the application. In a 3D display or a dual view display, the viewing zones must not overlap and create what is commonly called cross-talk. With privacy displays, the restricted viewing zone should have a sharp cut-off. Any leakage of light at angles outside the private zone could allow a prohibited person to read the display. As the quality of the light direction in such displays is important, the control of the light direction is done at the expense of power saving.
Some other displays provide a power saving benefit as a function of the user position. (Viewing Direction-Aware Backlight Scaling, Chih-Nan Wu and Wei-Chung Cheng, Great Lakes Symposium 2007 on VLSI, p 281-286) In this paper, the backlight power is turned up when a viewer moves away from the central axis. This compensates for the fact that the display's apparent brightness would otherwise be lower to an off-axis viewer. However these displays still send light in all directions and so they waste most of this light.